Nutrition is a key component of disease prevention; however, the health effects of certain foods, including sugar-sweetened beverages and meat, continue to be vigorously debated. A major factor limiting our ability to draw conclusive links between diet and health is the error and bias inherent in self-reported dietary intake; thus, objective biomarkers of diet are critically needed. This project will evaluate a new type of objective biomarkers of sugar-sweetened beverage (SSB), meat, and fish intake: molecular stable isotope profiles or ?MSIPs?. To generate MSIPs, naturally-occurring differences in stable isotope ratio will be measured in suites of individual molecules, including amino acids and fatty acids. Because many foods, including SSBs, meat and fish, differ naturally in both their stable isotope ratios and their molecular profiles, we should be able to improve the dietary specificity of each type of biomarker by measuring them in combination. This research leverages a 12-week, inpatient feeding study that is currently underway (55% complete) at the NIDDK Phoenix Epidemiology and Clinical Research Branch (PECRB). The ?Developing Biomarkers of Diet (DBD)? study was designed to test whole-tissue stable isotope biomarkers of long-term dietary exposure to SSB, meat and fish in blood plasma, red blood cells (RBC), hair and adipose tissue (#NCT01237093 at ClinicalTrials.gov). The treatment arms include all combinations of presence or absence of dietary SSB, meat and fish, allowing all three dietary effects as well as their interactions to be tested in an efficient design. The DBD study presents an ideal platform for assessing MSIPs as biomarkers of SSB, meat and fish intake and for measuring the time frame over which they change in plasma and RBC over the course of a 12 week dietary intervention. Specific Aim 1 will evaluate the validity of amino acid MSIPs as biomarkers of SSB, meat and fish intake in plasma, RBC and hair, and the time frame over which amino acid MSIPs change with diet in plasma and RBC. Specific Aim 2 will evaluate the validity of fatty acid MSIPs as biomarkers of SSB, meat and fish intake in plasma, RBC and adipose tissue, and the time frame over which fatty acid MSIPs change with diet in plasma and RBC. Biomarkers of diet based on stable isotope ratios have attracted growing interest; however, knowledge gaps remain, specifically related to molecule-specific isotopic measures. This research combines advanced analytical methodology with an unusually long and well-controlled feeding study to validate a promising new type of dietary biomarker for public health research.